Recording performance for magnetic disks is determined by three basic characteristics i.e. narrow PW50, high overwrite, and low noise. PW50 is the pulse width of the bits expressed in either time or distance, defined as the width of the pulse at half-maximum. Having a narrower (and more well-defined) pulse allows for higher recording density. A wide PW50 means that the bits are crowded together, causing them to interfere with each other. This interference is termed inter-symbol interference. Excessive inter-symbol interference limits the packing density of bits in a given area.
Conventionally, there are number of media factors which affect PW50. In order to achieve narrow PW50, the coercivity ("Hc") of the media must be high. However, if Hc is too high, the head field will have a difficult time saturating the media, resulting in poor overwrite. Overwrite ("OW") is a measure of the ability of the media to accommodate overwriting existing data. That is, OW is a measure of what remains of a first signal after a second signal (for example of a different frequency) has been written over it. OW is poor when a significant amount of the first signal remains. OW is generally affected by Hc, thickness, and the hysteresis loop squareness of the film.
PW50 may be reduced by using a thinner magnetic film. Another means of reducing PW50 is to increase hysteresis loop squareness, and narrow the switching field distribution, as described by William and Comstock in "An Analytical Model of the Write Process in Digital Magnetic Recording," A.I.P. Conf. Proc. Mag. Materials, 5, p. 738 (1971). Hysteresis loop squareness ("S") has several components, including coercivity squareness ("S*") and remnant coercivity squareness ("S*rem").
Noise performance of a magnetic film can be defined in terms of read jitter and write jitter. Noise, together with inter-symbol interference, contributes to the uncertainty in the location of the individual bits, which cause the data to be read with some displacement in timing from that which is expected. This displacement is referred to as bit shift. The bit shift needs to be reduced to a minimum for a given timing window of the bit in order to assure accuracy in reading the bit.
Read jitter is primarily determined by the amount of signal available from the bit, and the electronic noise in the channel. A thicker magnetic film will typically provide reduced read jitter. Unlike read jitter, write jitter is determined by the intrinsic noise of the film. Intrinsic media noise has been theoretically modeled by Zhu et al. in "Micromagnetic Studies of Thin Metallic Films", J. Appl. Phys., vol. 63, no. 8, p. 3248 (1988), which is incorporated by reference herein. Chen et al. describe the source of intrinsic media noise in "Physical Origin of Limits in the Performance of Thin-Film Longitudinal Recording Media," IEEE Trans. Mag., vol. 24, no. 6, p. 2700 (1988), which is also incorporated by reference herein. The primary source of intrinsic noise in thin film media is from the interparticle exchange interaction. In general, the higher the exchange interaction, the greater the noise.
The noise from interparticle exchange interaction can be reduced by isolating the individual particles. This may be accomplished by spacing the grains apart from one another, or by interposing a non-magnetic material or insulator at the grain boundaries, as described by Chen et al. in the aforementioned "Physical Origin of Limits in the Performance of Thin-Film Longitudinal Recording Media." The amount of separation needs to be only a few angstroms. There is another interparticle interaction, called magnetostatic interaction, which acts over a much greater distance between particles as compared to the exchange interaction. Reducing the magnetostatic interaction does reduce intrinsic media noise slightly. However, the effects of magnetostatic interaction actually improve hysteresis loop squareness and narrow the switching field distribution, and hence improve PW50 and OW. Therefore, magnetostatic interaction is generally tolerated.
In order to obtain the best performance from the magnetic media, each of the above criteria--PW50, noise and OW--must be optimized. This is a formidable task, as each of these performance criteria are interrelated. For example, obtaining a narrower PW50 by increasing the Hc will adversely affect OW, since increasing Hc degrades OW. A thinner media and lower remnant magnetization-thickness product ("MrT") yields a narrower PW50, however the read jitter increases because the media signal is reduced. Increasing squareness of the hysteresis loop contributes to narrower PW50 and better OW, but generally increasing squareness increases noise. Thus, the amount that PW50 may be narrowed is limited by the increase in noise. Providing a mechanism for separating or isolating the grains to break the exchange coupling can effectively reduce the intrinsic media noise. Noise is improved by eliminating the interparticle exchange interaction. A slight further reduction of noise is possible by reducing magnetostatic interaction, but this reduces the hysteresis loop squareness and increases the switching field distribution, which degrades PW50 and OW.
In order to obtain the optimum media performance, the remnant magnetization-thickness product ("MrT") must be reduced for better OW and PW50, but still retain sufficient signal to maintain acceptable read jitter. This is principally accomplished by reducing the film thickness, and using an alloy having a higher saturation magnetization ("Ms").
Therefore, an optimal thin film magnetic recording media for high density recording applications, i.e., that can support high bit densities, requires low noise without sacrificing the switching field distribution, S*, and S*rem. Recording density can then be increased since bit jitter is reduced. In order to achieve the best compromise in performance, the individual grains of the magnetic film must be isolated to eliminate the exchange interaction, and grains must be uniform and have a tight distribution of sizes to minimize intrinsic media noise while maintaining high hysteresis squareness.
One type of magnetic media which has allowed optimizing the above performance criteria is based on alloys of cobalt and platinum. Typical examples of such an alloy include CoPt, CoNiPt, CoCrPt, and CoNiCrPt. Attributes of Co--Pt alloys have been described by Murdock et al. in "Roadmap to 10 Gb/in.sup.2 Media: Challenges", IEEE Trans. Mag., 1992, page 3078, by Opfer et al. in "Thin Film Memory Disk Development", Hewlett-Packard Journal, November 1985, and by Aboaf et al., in "Magnetic Properties and Structure of Co--Pt Thin Film", IEEE Trans. Mag., page 1514, 1983, each incorporated herein by reference. Increasing storage capacity demands and performance requirements have motivated a search for ways to improve Co--Pt based alloys.
As mentioned by Yamashita et al. in U.S. Pat. No. 5,180,640, also incorporated herein by reference, typical Co--Pt alloys suffer from poor resistance to corrosion. This is of increasing concern as modern recording media are integrated into portable computers and specialized applications which are exposed to increasingly hostile environments such as extremely high or low temperatures, high humidity, etc. Therefore, the Co--Pt base is typically combined with corrosion resistant elements to enhance corrosion resistance while still providing the desired magnetic properties. For example, Howard et al., in U.S. Pat. No. 4,789,598, teaches the addition of greater than 17 atomic percent ("at. %") of the non-magnetic material chromium (Cr) to a Co--Pt alloy to obtain improved resistance to corrosion. However, the corrosion resistance obtained by adding such a large percentage of Cr to the Co--Pt alloy has a detrimental effect on a number of the magnetic properties of the thin film, for example a substantial reduction of the Ms of the film. As described above and otherwise well known, it is desirable to provide the highest possible Ms to allow the magnetic film to be kept as thin as possible to facilitate better magnetic field penetration and increased magnetic field gradient from the writing head due to the reduction in the spacing between the head and the magnetic layer, both of which are known to result in lower PW50 and better OW.
One approach to minimizing the detrimental effect of adding a large percentage of Cr to the Co--Pt alloy is to simply reduce the amount of Cr added. However, this approach is limited by the at. % of the non-magnetic material required to obtain the desired degree of corrosion resistance. For example, as taught by Yamashita et al. in U.S. Pat. No. 5,180,640, the corrosion resistance drops sharply for less than 10 at. % of Cr in the CoCrPt alloys. In addition, SNR also decreases with decreasing Cr percentage, as taught by K. E. Johnson et al. in "Composition Effects on Recording Properties of CoPtCr Thin Film Media" IEEE Trans. Mag., MAG-29, vol. 6, page 3670 (1993).
Another approach to minimizing the detrimental effect of adding a large percentage of Cr to the Co--Pt alloy is to introduce a combination of nickel (Ni) and Cr. This results in enhanced corrosion resistance while maintaining a higher Ms, since Ni is ferromagnetic. This approach is disclosed in the aforementioned U.S. Pat. No. 5,180,640 of Yamashita et al., which teaches adding 3 to 8 at. % Ni together with 5 to 10 at. % Cr in combination to form the quaternary alloy CoNiCrPt. This alloy has been shown to provide good corrosion resistance and higher Ms than that taught by Howard et al. However, it is desirable to further reduce or, if possible, completely eliminate the Cr content of the alloy.
A number of references have suggested adding tantalum (Ta) and/or boron (b) to the magnetic layer to improve corrosion resistance. For example, the Japanese patent specifications of Hitachi nos. JP05290352-A and JP05314468-A each discuss the addition of Ta. While it is inferred from these two references that the addition of Ta serves to improve corrosion resistance, there is no mention in either reference of the negative aspects of the addition of Cr. In fact, the alloys taught by each of these references include Cr in addition to Ta.
Another reference discussing the addition of Ta is Furusawa et al. in U.S. Pat. No. 4,950,548, teaches the addition of tantalum and/or titanium (Ti) for the purpose of imparting corrosion resistance to the alloy. However, none of the references have demonstrated an alloy with any specific combination of tantalum and titanium which simultaneously provide improved corrosion resistance, higher coercivity, higher saturation magnetization, and higher squareness.
With recording density increasing at a rate of about 30 to 60% per year, and with disk drives becoming increasingly portable, magnetic media are expected to simultaneously have increasingly higher corrosion resistance and improved magnetic performance. A higher Ms alloy for the magnetic media not only enhances recording performance but also reduces the cost of the disk--a thinner film is deposited which enhances the throughput and utilization time of the deposition system. The benefits of enhanced coercivity and squareness are also well known.